It will be appreciated that optical alignment systems have included flexible hinge type mechanisms for positioning the optical axis of an optical element through flexure of live hinges. By live hinges, what is meant is a necked-down area on a solid object, being made thin enough that it forms a flexure, allowing one solid portion of the object to move with respect to the solid portion on the opposite side of the necked down region. This movement is pure rotation about the hinge, but for a small angle approximates linear motion. Usually, the elastic limit of the material from which the object is made will allow a few degrees of motion before plastic deformation occurs. Sometimes plastic deformation of the live hinge is allowed, and causes no problem, as long as repeated adjustments are not to be made. The typical deployments of these types of hinges are not particularly well suited for harsh environments because they are usually designed for use in a laboratory, in which the conditions are carefully controlled as to temperature, humidity and mechanical stress. Usually these elements are made up of multiple materials of different thermal coefficients of expansion and generally do not have lock down mechanisms. If they do, the lock down mechanisms deleteriously affect the previously made adjustments. Moreover, laboratory alignment fixtures are not sufficiently robust when used outside the laboratory such as for laser target designators, communications systems involving lasers or in surveying equipment. Additionally, unless ruggedized and specially designed, laboratory fixturing apparatus for the alignment of optical elements is unacceptable for military and commercial applications because optical alignment is severely affected by changing environmental conditions.
For instance, if a laser target designator is mounted on an aircraft, when the aircraft is in the vicinity of a target, the designator is utilized to mark the target by illuminating the target with laser radiation. The way this is usually accomplished is to first obtain a visual image of the target, with the target centered in the crosshairs of a screen. It is then incumbent upon the target acquisition system to be able to position a laser beam exactly along the optical axis that resulted in the visual image. In this manner the visual image is co-boresighted with the laser axis, or visa versa.
In tactical situations, target acquisition requires accuracy of approximately plus or minus three feet in order to be able to accurately direct an ordinance to a particular target. If the laser designated spot is displaced by a significant amount, for instance 20 feet, then the ordinance may miss its target altogether. Thus it is possible that either a tank will not be properly illuminated or that a missile silo can be missed.
In one operating scenario, the aircraft stands off from the target, for instance, one mile and illuminates the appropriate target by providing that the crosshairs on the visual image corresponds to the portion of the target to be illuminated. The plane then executes a high G roll and flies away from the target with the laser target designating equipment tracking the target as the aircraft executes its maneuvers.
It will be appreciated that aside from the system dynamics of the laser target designator, the precision by which the laser is co-boresighted with the optical image depends in large part upon how accurately the system was aligned to begin with. This means, for instance, when using one laser beam to end-pump a small crystal of another laser, such as a designator laser, the pump beam must be accurately focused into the lasing material. If the pump laser is mounted remotely from the designator laser compartment, the output of the pump laser can be transmitted through a fiber optic link and focused on the pump region, typically in a 0.1 millimeter or less wide volume within the laser rod.
This focusing operation is highly critical, with any off centering of the pumped radiation in the laser rod causing a loss of laser power and boresight alignment.
The pump laser is mounted remotely because one does not want to introduce excess waste heat in the region of the designator laser cavity or compartment. The heat generated by the pumping laser is sometimes referred to as self-contamination, which can cause a shift of the optical elements resulting in boresight error.
Typically in such an environment, it is important to be able to accurately position the distal end of the optical fiber relative to a collector lens and a focusing lens assembly so that light from the distal end of the fiber is appropriately focused in the aforementioned pump region of the laser rod. So precise is the positioning requirement for the end of the fiber optic cable that lateral movement of the cable of greater than one half-micron results in reduced laser power, and possibly boresight error, due to movement of the focused radiation, and thus the pump region within the laser rod. For a two-millimeter laser rod, the pump region is typically on the order of 0.1 millimeter in diameter.
Even with the optical fiber termination being appropriately positioned during an alignment procedure, if due to the aforementioned harsh environment lateral positioning errors build up and exceed the one half-micron limit, then the laser beam which creates the laser designator spot can be off by as much as 20 feet at a standoff distance of one mile.
As mentioned, a laser beam displaced by 20 feet can result in a completely missed target or the illumination, sometimes referred to as painting, of an unintended target.
From the point of view of laser target designators, the pointing accuracy of the entire system typically must be less than 0.5 milliradians. While there are indeed many factors in the laser target designation system which can contribute to boresight error, it is incumbent upon the laser itself to contribute as little as possible to this error. The laser, as a component of a designator system may only be allowed as much as 0.2 milliradians of boresight error, and a 10% energy drop over the entire environmental range of changing temperatures, pressures, accelerations, self contamination loading, and acoustics.
Another application that requires the ultimate in boresight accuracy is in the area of countermeasures. In these types of environments an incoming missile is to be countered. This requires firing a modulated laser beam towards the incoming missile with such accuracy that the beam impinges on the missile as it approaches its target. Sometimes the time window for acquisition and beam deployment is less than two seconds. Moreover there must be enough laser power illuminating the missile to counter it. This requires highly accurate aiming, which cannot be deleteriously affected by misalignments of the optical elements on the optical bench. Such countermeasures require the same alignment accuracies as discussed above, the 0.5 milliradian accuracy.
There are however other applications for an X, Y positioning fixture or unit for optical elements, not the least of which is when one is trying to couple the ends of opposed single mode fibers. A single mode fiber, such as might be used in a fiber optic telecommunications system, is commonly less than ten microns in diameter. Assuming that the two fibers are to be aligned along the same optical axis, one would commonly require having a one half-micron transverse alignment accuracy and stability in order to successfully couple one fiber to the other with less than 0.4 dB splice loss. Certainly, there are other applications where the alignment requirements are even tighter, and where the precision alignment must be maintained in harsh environments involving large temperature swings as well as high dynamic and static loading conditions.
Moreover, for optical communications it is important that the laser source illuminate the intended receiver. When, for instance, providing laser communications between an over flying satellite and an earth station, any positioning errors will affect the receipt of information contained on the beam. Free space laser communications devices are becoming more important in commercial and military applications. To keep communications secure, narrow beam divergences and accurate pointing will be necessary qualities of these systems. In this scenario, as well as those presented above, boresight alignment is critical. In order to achieve the necessary stringent tolerances, the optical elements themselves must be accurately aligned and must maintain their alignment. There is therefore a need for a robust device for achieving alignment and preserving it in harsh environments.